A vast amount of information has accumulated in recent years regarding the role of calcium (Ca ion2), as an obligatory mediator of the secretory process. Nevertheless, the mechanism by which Ca ion 2 exerts its critical action is still an enigma. Our approach to elucidating this unknown role of Ca ion 2 involves a study of its interaction with other putative mediators, namely, cyclic AMP, cyclic GMP, and the prostaglandins (PGs); for although there is also a great deal of information concerning the possible functions of these substances in a number of secretory organs, including the adrenal cortex, there is at present an obvious gap in our knowledge of their interactions. Hence, the prime thrust of this investigation is to probe the role of calcium in the mechanism of action of ACTH on isolated adrenocortical cells by a comprehensive study of its interplay with PGs and cyclic nucleotides. It is anticipated that the functional relationships developed as a result of this investigation will help to define the role of Ca ion 2 in ACTH-induced steroid production and release; furthermore, it may provide deeper insight into the fundamental mechanisms which control secretory processes, in general.